This invention relates to processes that generate a gas stream comprising ammonia (NH3) and carbon dioxide (CO2). In particular, the invention relates to a method for reducing undesirable side products that form when ammonia is removed and recovered from the gas stream.
An example of a process that generates such a gas stream is the Andrussow method for preparing hydrogen cyanide (see U.S. Pat. No. 1,934,838), ammonia, an oxygen-containing gas such as air, and hydrocarbon gases such as methane are fed to a reaction system at ambient or elevated temperature. The reactants are then reacted in the presence of a platinum-containing catalyst at temperatures of 1000xc2x0 C. to 1400xc2x0 C. to produce hydrogen cyanide. A portion of the hydrocarbon/ammonia reactant feed gas is combusted to provide the energy required to maintain the highly endothermic cyanide formation reaction.
One problem associated with the Andrussow process is that there is a high level of residual ammonia in the exit gas. The residual ammonia must be removed from the HCN product stream to avoid polymerization of HCN. Although low levels of ammonia in the exit gas may be neutralized with acid in a purification process, the ammonia concentration in the exit gas of the Andrussow process is too high for the HCN product stream to be sent directly to such a neutralization process. Therefore, the exit gas containing residual ammonia must first be sent to a separate process for removing the majority of the ammonia and then the product stream sent to a purification process.
Ammonia can be removed from a gas stream by contacting the stream with a mineral acid complex, derived from acids such as nitric, phosphoric, or boric to chemically capture the ammonia. The aqueous ammonia/acid complex is then thermally decomposed to free the ammonia, which can be recovered for use in a downstream process (see U.S. Pat. No. 2,797,148 for example).
Another problem in downstream processes, such as Andurssow HCN processes, is that they are susceptible to contaminants produced during the operation of the ammonia recovery process. A contaminant that is of particular concern is iron oxide, an abrasive material that causes premature wear on equipment and may also lower downstream-process catalyst efficiency. The present invention is a series of preventive steps which alleviate iron oxide contamination of processes downstream of ammonia recovery processes.
The applicants have discovered that it is possible to significantly reduce iron oxide contamination in an ammonia recovery process by preventing ammonium carbamate from depositing on the inside walls of piping and associated equipment within the ammonia recovery process and by using piping and equipment materials that are not susceptible to corrosion by ammonium carbamate. One aspect of the invention is a method for reducing iron oxide contamination of the downstream process by physically removing iron oxide and iron oxide precursor compounds from the process stream. In a further aspect of the invention, the pipes and equipment carrying recovered ammonia from an ammonia recovery process are heated to prevent ammonium carbamate from depositing on the inside walls of the pipes and equipment. In a still further aspect of the invention, the process piping and associated equipment carrying recovered ammonia from an ammonia recovery process is constructed from a material that is not susceptible to corrosion by ammonium carbamate.
FIG. 1 depicts a simplified flow diagram of an embodiment of the present invention.
FIG. 2 depicts a simplified drawing of an embodiment of an apparatus for separating liquids, colloids and particulates from a gas stream.